1. Field of the Invention
The present invention relates to fasteners and more particularly, but not by way of limitation, to a frusto-conical recess and associated driving tool having a torque limiting feature and a shaped recess to facilitate mating the driver with the recess.
2. Description of the Prior Art
The extensive use of screw-type fasteners in the production and maintenance of vehicles and particularly aircraft requires that the fastener design be as close to perfection as possible. Many thousands of manhours are wasted each year in aircraft maintenance alone in trying to remove "frozen" screws that have become stuck due to the duration of installation or most ususally from having been over torqued during installation.
Another problem inherent in the aircraft screw-type fastener is that upon application of excessive torque, the metal fastener is often damaged. The fastener recess is typically designed to insure that adequate installation torque may be applied with little thought given to the subsequent requirement of removing the installed fastener.
Another problem giving rise to damaged fastener recesses is that tolerances between the recess and the driver tool often result in uneven torque loads being applied to the wrenching elements of the recess. This problem causes "point loads" to be applied to the walls of the recess at its outer extremities resulting in failure of the recess normally called "camming out". When this problem occurs, the only remedy is to drill out the screw.
The greatest danger in drilling out a screw is that of improper alignment with the screw shank causing the drill to exit the screw recess hole damaging the installed component. The component must then be replaced or, if possible, be redrilled to accept an oversized screw which causes obvious subsequent problems.